This application is a 371 of PCT/EP99/03319, which was filed on May 14, 1999.
The present invention relates to the preparation of nanoscale transition metal or alloy colloids having a high dispersibility in different solvents, to the colloids thus obtained and their use.
Nanoscale transition metal or alloy colloids are of technical importance as precursors of homogeneous and heterogeneous chemical catalysts, as catalysts in fuel cell technology, further as materials for coating surfaces (especially in lithography and in touch-sensing technology), as ferrofluids, e.g., in vacuum-tight rotational bushings, in active vibration dampers (automobile construction), and in tumor control using magnetically induced hyperthermia. They further serve as starting materials in sol/gel technology.
The technically advantageous universal use of nanostructured monometal and multimetal particles requires the decomposition-free redispersibility of the metal particles in a high metal concentration in a wide range of hydrophobic and hydrophilic solvents including water.
There have been many attempts to selectively change the dispersing properties of nanoscale transition metal or alloy colloids. Thus, G. Schmid et al. and C. Larpent et al. as well as N. Toshima et al. describe the conversion of hydrophobic metal colloids to water-soluble colloid systems by exchanging hydrophobic with hydrophilic protective shells through extractive ligand exchange at the interface between the organic and aqueous phases [e.g., G. Schmid et al., Polyhedron Vol. 7 (1988) p. 605-608; G. Schmid, Polyhedron Vol. 7 (1988) p. 2321; C. Larpent et al., J. Mol. Catal., 65 (1991) L 35; N. Toshima et al., J. Chem. Soc., Chem. Commun. (1992), p. 1095]. However, this kind of protective shell exchange allows only for the replacement of hydrophobic by hydrophilic ligands and vice versa, but does not enable the decomposition-free redispersibility of the metal particles in a high metal concentration in a wide range of hydrophobic and hydrophilic solvents including water. Thus, the problem of repeptization of nanoscale transition metal or alloy colloids in any solvents cannot be solved by ligand exchange. For the stabilization of metal, metal oxide and metal sulfide colloids, Antonietti et al. (PCT/EP 96/00721, WO 96/26004) use block copolymers as micelle builders in organic (e.g., toluene, cyclohexane, THF) or inorganic solvents (e.g., water, liquid ammonia). The nature of the respective side chains of the micelles restricts the solubility of the colloids to either organic or inorganic media. Thus, this way does not enable a broad solubility range either.
Chagnon (U.S. Pat. No. 5,147,573) describes the preparation of electrically conducting superparamagnetic colloidal dispersions from solid magnetic particles by adsorptive coating with (water-stable) organometallics, e.g., Sn(C2H5)4, in water, followed by reaction with dispersing aids (e.g., surfactants) and addition of an organic carrier liquid, such as toluene. This method does not result in isolatable metal colloids and is not applicable to precious metals (see Comparative Example 4).
It has been the object of the present invention to provide a process which overcomes the above mentioned difficulties and enables the selective modification of the dispersing properties of nanoscale transition metal or alloy colloids for a decomposition-free repeptization of the colloids, modified and isolated with retention of the size distribution, in any desired hydrophobic or hydrophilic solvents including water for further technical processing in as high as possible a concentration.
It has now been found that colloids which are dispersible in a wide range of hydrophobic and hydrophilic solvents including water are formed by reading reactive metal-carbon bonds in the protective shell of organometallic-prestabilized transition metal or alloy colloids, prepared by known synthetic methods, of metals of Periodic Table groups 6 to 11 [e.g., K. Ziegler, Brennstoffchemie 35 (1954) p. 322, cf. K. Ziegler, W. R. Kroll, W. Larbig, O. W. Steudel, Liebigs Annalen der Chemie, 629 (1960) p. 74, and Houben-Weyl, Methoden der organischen Chemie, E. Mxc3xcller (ed.), Volume 13/4, Thieme Verlag Stuttgart (1970) p. 41; J. S. Bradley, E. Hill, M. E. Leonowic, H. Witzke, J. Mol. Catal. 41 (1987) p. 59-74; J. Barrault, M. Blanchart, A. Derouault, M. Kisbi, M. I. Zaki, J. Mol. Catal. 93 (1994) p. 289-304] or of organometallic-prestabilized and organometallic-pretreated transition metal or alloy colloids (Periodic Table groups 6 to 11) presynthesized by known synthetic methods [e.g., J. S. Bradley, Clusters and Coloids, Ed.: G. Schmid, VCH Weinheim (1994) p. 459-536], hereinafter referred to as starting materials, with a chemical modifier. Suitable chemical modifiers include materials capable of protolysis of metal-carbon bonds [cf. F. A. Cotton, G. Wilkinson; Advanced Inorganic Chemistry, John Wiley and Sons, New York, 4th ed. (1980) p. 344; Ch. Eischenbroich, A. Salzer; Organometallchemie, B. G. Teubner, Stuttgart (1986) p. 93] or of insertion of C/C, C/N or C/O multiple bonds in metal-carbon bonds [G. Wilkinson, F. G. A. Stone; Comprehensive Organometallic Chemistry, Vol. 1, Pergamon Press, Oxford (1982) p. 637, p. 645, p. 651] or of Lewis acid-base interactions with metal carbon bonds [Ch. Elschenbroich, A. Salzer; B. G. Teubner, Stuttgart (1986) p. 95; G. Wilkinson, F. G. A. Stone; Comprehensive Organometallic Chemistry, Vol. 1, Pergamon Press, Oxford (1982) p. 595].
The starting materials can be prepared by reacting metal salts, halides, pseudohalides, alcoholates, carboxylates or acetylacetonates of metals of Periodic Table groups 6 to 11 with protolyzable organometallic compounds. Alternatively, for preparing the starting materials, colloids of transition metals of Periodic Table groups 6 to 11 synthesized by other methods, e.g., precious-metal anticorrosion-protected colloids of Fe, Co, Ni or their alloys, may be reacted with organometallic compounds. The protective shell of the thus prepared colloidal starting materials contains reactive metal-carbon bonds which can react with the modifiers (see Example 1, protolysis experiment). Non-colloidal solid metal particles or powders (cf. Chagnon, U.S. Pat. No. 5,147,573) cannot be reacted by the process according to the invention (Comparative Examples 1, 2 and 3). Suitable organometallic compounds include protolyzable organoelement compounds of metals of Periodic Table groups 1 or 2 and 12 and 13.
Suitable chemical modifiers with which these organometallic-prestabilized starting materials are reacted to achieve a high dispersibility (at least 20 mmol of metal per liter, preferably  greater than 100 mmol of metal per liter) include, for example, alcohols, carboxylic acids, polymers, polyethers, polyalcohols, polysaccharides, sugars, surfactants, silanols, active charcoals, inorganic oxides or hydroxides. A particular characteristic of the modification process according to the invention is the retention of particle size.
According to the invention, the reaction of the organometallic-prestabilized starting materials with such modifiers may also be effected in situ, i.e., without intermediate isolation of the starting materials.
As determined by elemental analysis (cf., e.g., Example 9), the protective shells of the transition metal or alloy particles modified according to the invention consist of metal compounds of the modifier with the elements of the organometallic compounds employed for prestabilization (Periodic Table groups 1 or 2 and 12 and 13, for example, Al or Mg; cf. Table 3, Nos. 18, 19, 24, 26, 29 and 30).
The modification process performed according to the invention permits the preparation of novel nanostructured transition metal or alloy colloids the dispersing properties of which are tailored to match the respective intended technical use. For example, the modification according to the invention of the organoaluminum-prestabilized Pt colloid used as the starting material (Table 1, No. 22) with polyoxyethylene sorbitan monopalmitate (Tween 40, Table 2, No. 15) yields a novel Pt colloid with a very wide dispersing range which can be redispersed both in lipophilic solvents, such as aromatics, ethers and ketones, and in hydrophilic media, such as alcohols or pure water, in concentrations of  greater than 100 mmol of Pt per liter without precipitation of metal (Table 3, No. 20).
In contrast, the modification according to the invention of the same organoaluminum-prestabilized Pt colloid used as the starting material with decanol or oleic acid (Table 2, Nos. 1 and 3) yields a Pt colloid with excellent redispersibility especially in engineering pump oils (Table 3, Nos. 7 and 9). The modification according to the invention of the same starting material with polyethylene glycol PEG 200, polyvinyl pyrrolidone, surfactants of the cationic, anionic or non-ionic-types or with polyalcohols, e.g., glucose (Table 2, Nos. 5-7, 9-11, 13 and 14), yields Pt colloids with excellent dispersing properties predominantly in aqueous media (Table 3, Nos. 10-12, 14-16, 18-20).
The dispersing properties of organoaluminum-prestabilized Fe bimetallic colloids can also be selectively adapted to their intended technical use by means of the modification according to the invention: Thus, the reaction of the Fe2Co organosol used as the starting material (Table 1, No. 34) with decanol (Table 2, No. 1) results in colloidal Fe2Co with advantageous dispersibility in special pump oils (Shell Vitrea Oil 100, Shell) as employed in-technical magnetic fluid seals (Table 3, No. 27). According to the invention, the organoaluminum-treated presynthesized Fe/Au organosol (Example 13, MK 41) as a starting material can be converted by modification with polyethylene glycol dodecyl ether to a hydrosol which can be redispersed without decomposition in physiologically relevant media, such as ethanol/water mixtures (25/75 v/v), in a high concentration ( greater than 100 mmol of metal per liter) (Table 3, No. 28).
The modification according to the invention of the organoaluminum-prestabilized Pt/Ru colloid used as the starting material (Table 1, No. 36) and having an average particle size of 1.3 nm as determined by TEM (transmission electron microscopy) with polyethylene glycol dodecyl ether yields a novel Pt/Ru colloid having the same average particle size of 1.3 nm as determined by TEM and being equally well dispersible in aromatics, ethers, acetone, alcohols and water (Example 11, Table 3, No. 29). As determined by TEM, the modification process according to the invention of the protective shell is effected with full retention of particle size even for very small particles.
Nanoscale transition metal or alloy colloids having protective shells modified according to the invention can be employed to technical advantage as precursors for the preparation of homogeneous and heterogeneous chemical catalysts. Nanoscale Pt or Pt alloy colloids having an average particle diameter of  less than 2 nm as determined by TEM (Examples 11 and 12, Table 3, Nos. 29 and 30) are suitable precursors for fuel cell catalysts. Nanoscale Fe, Co, Ni or alloy colloids (Examples 3 and 10, Table 3, Nos. 2 to 4 and 27) and gold-protected Fe (Example 13, Table 3, No. 28), Co, Ni or alloy colloids are employed in the magneto-optical storage of information and as magnetic fluids in magnetic fluid seals. Fe colloids (Example 13, Table 3, No. 2) and gold-protected Fe colloids (Example 13, Table 3, No. 28) serve as magnetic cell markers and for magnetic cell separation. Fe colloids (after treatment with oxygen, if necessary) and gold-protected Fe colloids with modified protective shells have fields of application in medical tumor therapy (magnetic fluid hyperthermia). Nanoscale transition metal or alloy colloids, especially of platinum, are employed as metallic inks in ink-jet printers and for laser sintering, for example, by coating quartz plates with the sol and sintering the dried layers with a CO2 laser to give a conductive metallic layer. Further, nanoscale transition metal or alloy colloids modified according to the invention are suitable for the coating of surfaces and for use in sol-gel processes.